Application of a multimedia activity model for evaluating the fate of persistent mobile organic chemicals in soil: The influuence of media-specific volume fractions

In recent years there has been increasing interest to screen and prioritize chemicals used in commerce with respect to their potential to contaminate drinking water sources.  The German Environment Agency (Umweltbundesamt – UBA), for instance, has proposed screening criteria to identify chemicals that may pose hazards to drinking water.  The hazard criteria aim to prioritize chemicals that are persistent, mobile and toxic (PMT) or very persistent and very mobile (vPvM).  The development and application of exposure assessment tools would  thus support prioritization activities.  This study summarizes the development of a multimedia environmental fate and transport model, which supports the exposure assessment of persistent mobile organic chemicals (PMOCs) under a variety of scenarios, including under steady-state, equilibrium and non-steady-state and non-equilibrium conditions. The developed model includes multilayer soil and aquifer compartments (unsaturated and saturated), which is used to enable a screening level estimate of exposure to PMOCs. Model results, based on an assumption of equilibrium and steady-state, provide an illustrative example regarding the relative importance of including a saturated compartment when estimating the concentration of PMOCs in groundwater that might be used for drinking water. Results demonstrate that the higher volume fraction of water in the saturated zone, relative to the unsaturated soil, causes the concentration of chemicals with properties consistent with PMOCs to be greater in the saturated zone.  Inclusion of a saturated zone, thus, represents an important refinement to screening-level multimedia models that might be used to estimate concentrations of PMOCs in groundwater.